1. Field of the Invention
The present invention relates to a polishing apparatus for polishing a workpiece such as a semiconductor wafer to a flat mirror finish, and more particularly to a polishing apparatus having a workpiece carrier, i.e., a top ring which is tiltable so as to follow undulation or surface irregularities of a polishing surface on a turntable.
2. Description of the Related Art
Recent rapid progress in semiconductor device integration demands smaller and smaller wiring patterns or interconnections and also narrower spaces between interconnections which connect active areas. One of the processes available for forming such interconnection is photolithography. Though the photolithographic process can form interconnections that are at most 0.5 .mu.m wide, it requires that surfaces on which pattern images are to be focused by a stepper be as flat as possible because the depth of focus of the optical system is relatively small. Conventionally, as apparatuses for planarizing semiconductor wafers, there have been used a self-planarizing CVD apparatus, an etching apparatus or the like, however, these apparatuses fail to fully planarize semiconductor wafers. Recently, attempts have been made to use a polishing apparatus for planarizing semiconductor wafers to a flatter finish with more ease than those conventional planarizing apparatus.
Conventionally, a polishing apparatus has a turntable and a top ring which rotate at respective individual speeds. A polishing cloth is attached to the upper surface of the turntable. A semiconductor wafer to be polished is placed on the polishing cloth and clamped between the top ring and the turntable. An abrasive liquid containing abrasive grains is supplied onto the polishing cloth and retained on the polishing cloth. During operation, the top ring exerts a certain pressure on the turntable, and the surface of the semiconductor wafer held against the polishing cloth is therefore polished by a combination of chemical polishing and mechanical polishing to a flat mirror finish while the top ring and the turntable are rotated. This process is called Chemical Mechanical polishing.
If the semiconductor wafer is not pressed against the polishing cloth under forces which are uniform over the entire surface of the semiconductor wafer, then the semiconductor wafer tends to be polished insufficiently or excessively in local areas depending on the applied forces. The following arrangements have been proposed in the art to prevent the semiconductor wafer from being pressed against the polishing cloth under irregular forces.
1) One conventional solution has been to apply an elastic pad of polyurethane or the like to a workpiece holding surface of the top ring for uniformizing a pressing force applied from the top ring to the semiconductor wafer.
2) According to another solution, the top ring, i.e., a workpiece carrier for holding a semiconductor wafer is tiltable with respect to the surface of the polishing cloth.
3) Still another attempt has been to press a region of the polishing cloth surrounding the semiconductor wafer, independently of the semiconductor wafer, for thereby eliminating an appreciable step between a region of the polishing cloth pressed by the semiconductor wafer and the surrounding region thereof.
FIG. 8 of the accompanying drawings shows a conventional polishing apparatus. As shown in FIG. 8, the conventional polishing apparatus generally comprises a turntable 5 with a polishing cloth 6 attached to an upper surface thereof, a top ring 1 for holding a semiconductor wafer 4 to be polished and pressing the semiconductor wafer 4 against the polishing cloth 6, and an abrasive liquid supply nozzle 25 for supplying an abrasive liquid Q to the polishing cloth 6. The top ring 1 is connected to a top ring shaft 8 and has an elastic pad 2 of polyurethane or the like attached to its lower surface. The semiconductor wafer 4 is held by the top ring 1 in contact with the elastic pad 2. The top ring 1 also has a cylindrical retainer ring 3 mounted on an outer circumferential edge thereof for preventing the semiconductor wafer 4 from being disengaged from the lower surface of the top ring 1. The retainer ring 3 has a lower end projecting downwardly from the lower surface of the top ring 1 for holding the semiconductor wafer 4 on the elastic pad 2 against disengagement from the top ring 1 under frictional engagement with the polishing cloth 6 during a polishing process.
In operation, the semiconductor wafer 4 is held against the lower surface of the elastic pad 2 which is attached to the lower surface of the top ring 1. The semiconductor wafer 4 is then pressed against the polishing cloth 6 on the turntable 5 by the top ring 1, and the turntable 5 and the top ring 1 are rotated independently of each other to move the polishing cloth 6 and the semiconductor wafer 4 relatively to each other, thereby polishing the semiconductor wafer 4. The abrasive liquid Q comprises an alkaline solution containing abrasive grains of the fine particles suspended therein, for example. The semiconductor wafer 4 is polished by a composite action comprising a chemical polishing action of the alkaline solution and a mechanical polishing action of the abrasive grains.
A spherical bearing 7 is slidingly interposed between the upper end surface of the top ring 1 and a lower end surface of the top ring shaft 8 which rotates the top ring 1. Specifically, the lower end surface of the top ring shaft 8 has a semispherical recess 8a defined centrally therein, and the upper end surface of the top ring 1 has a semispherical recess 1b defined centrally therein. The spherical bearing 7 is slidingly received in the semispherical recesses 8a and 1b. Even if the upper surface of the turntable 5 is slightly inclined, the top ring 1 is tilted with respect to the top ring shaft 8 by the spherical bearing 7. The top ring shaft 8 has a plurality of torque transmitting pins 107 extending radially outwardly and held in point-to-point contact with a plurality of respective torque transmitting pins 108 projecting upwardly from the upper end surface of the top ring 1. Therefore, even when the top ring 1 is tilted, the torque can reliably be transmitted from the top ring shaft 8 to the top ring 1 through the point-to-point contact between the torque transmitting pins 107 and 108.
FIG. 9 of the accompanying drawings shows a polishing apparatus which has been proposed by the applicant of the present invention in Japanese patent application No. 7-287976. As shown in FIG. 9, a semiconductor wafer 4 is held by a top ring 1 and pressed against a polishing cloth 6 on a turntable 5. The semiconductor wafer 4 is retained on the top ring 1 by a cylindrical retainer ring 3 which is disposed around and connected to the top ring 1 by keys 18. The keys 18 allow the retainer ring 3 to move vertically with respect to the top ring 1 and to rotate together with the top ring 1. The retainer ring 3 is rotatably supported by a bearing 19 which is held by a bearing holder 20 operatively coupled by a plurality of (e.g. three) circumferentially spaced shafts 21 to a plurality of (e.g. three) circumferentially spaced retainer ring air cylinders 22. The retainer ring air cylinders 22 are fixedly mounted on a top ring head 9. The top ring 1 has an upper surface held in sliding contact with a spherical bearing 7 that is slidably supported on the lower end of a top ring shaft 8. The top ring shaft 8 is rotatably supported by the top ring head 9. The top ring 1 is vertically movable by a top ring air cylinder 10 mounted on the top ring head 9 and operatively connected to the top ring shaft 8.
The top ring air cylinder 10 and the retainer ring air cylinders 22 are connected to a compressed air source 24 respectively through regulators R1 and R2. The regulator R1 regulates the air pressure supplied from the compressed air source 24 to the top ring air cylinder 10 to adjust the pressing force for pressing the semiconductor wafer 4 against the polishing cloth 6 by the top ring 1. The regulator R2 regulates the air pressure supplied from the compressed air source 24 to the retainer ring air cylinders 22 to adjust the pressing force for pressing the retainer ring 3 against the polishing cloth 6. By adjusting the pressing force of the retainer ring 3 with respect to the pressing force of the top ring 1, the distribution of polishing pressures is made continuous and uniform from the center of the semiconductor wafer 4 to its peripheral edge and further to the outer circumferential edge of the retainer ring 3 disposed around the semiconductor wafer 4. Consequently, the peripheral portion of the semiconductor wafer 4 is prevented from being polished excessively or insufficiently. The semiconductor wafer 4 can thus be polished to a high quality and with a high yield.
FIG. 10 of the accompanying drawings illustrates the states of the semiconductor wafer 4, the polishing cloth 6, and the elastic pad 2 while the semiconductor wafer 4 is being polished by a conventional polishing apparatus. When only the semiconductor wafer 4 is pressed against the polishing cloth 6, the peripheral portion of the semiconductor wafer 4 is positioned at a boundary between contact and noncontact with the polishing cloth 6 and also at a boundary between contact and noncontact with the elastic pad 2. Therefore, at the peripheral portion of the semiconductor wafer 4, the polishing pressure applied to the semiconductor wafer 4 by the polishing cloth 6 and the elastic pad 2 is not uniform, thus the peripheral portion of the semiconductor wafer 4 is liable to be polished to an excessive degree. As a result, the peripheral edge of the semiconductor wafer 4 is often polished into an edge-rounding. The polishing apparatus shown in FIG. 9 is effective to reduce the excessive polishing of the peripheral edge of the semiconductor wafer 4 because the retainer ring 3 is pressed against the polishing cloth 6 around the semiconductor wafer 4, independently of the top ring 1.
Another proposed retainer ring comprises an annular weight vertically movably disposed around the top ring for pressing the polishing cloth by its own weight.
In the conventional polishing apparatus, and also the polishing apparatus shown in FIG. 9, which has been proposed in Japanese patent application No. 7-287976, the top ring is tiltable to keep the surface to be polished of the semiconductor wafer parallel to the polishing cloth even if the polishing cloth has undulations or other surface irregularities. The top ring is made tiltable by a spherical cup bearing or a ball slidably interposed between the top ring and the top ring shaft. One problem of the tiltable top ring is that the top ring may be tilted excessively due to smooth sliding motion, and cannot be kept in parallel to the polishing cloth 6. In other words, the tiltable top ring cannot be stably held in its desirable posture with respect to the polishing cloth.
The above problem will be described in detail below with reference to FIG. 11 of the accompanying drawings. In FIG. 11, the top ring 1 is pressed against the polishing cloth 6 under a pressing force F.sub.1, and the retainer ring 3 is pressed downwardly substantially parallel against an upper surface 6a of the polishing cloth 6 under a pressing force F.sub.2 by the retainer ring air cylinders 22. The top ring 1, which is positioned inside of the retainer ring 3, is tiltable with respect to the polishing cloth 6 by the spherical bearing 7 slidably interposed between the top ring shaft 8 and the top ring 1. When the semiconductor wafer 4 held by the top ring 1 is polished, the top ring 1 is liable to be tilted due to frictional forces between the lower surface of the semiconductor wafer 4 and the upper surface 6a of the polishing cloth 6. If the top ring 1 is excessively tilted out of parallelism with the polishing cloth 6, then the semiconductor wafer 4 held on the top ring 1 cannot be polished to a flat mirror finish.